Method to detect surface metal contamination

ABSTRACT

A method for detecting surface or near surface metal contamination in a semiconductor or silicon structure is described in which the structure or a part thereof is exposed to an excitation beam of predetermined wavelength and collecting luminescence from the structure in as the form of PL map having a substantially uniform PL intensity level provided by the semiconductor; and inspecting the map for one or more regions of enhanced PL intensity identifying characteristic surface or near surface metal contamination. In particular, the method is applied as an in-process quality control or as a quality control of processed structures such as interconnects.

This is a nationalization of PCT/GB01/04454, filed Oct. 5, 2001 andpublished in English.

The invention relates to a non-destructive method and to the use of anapparatus for detecting surface metal contamination in semiconductorssuch as silicon and particularly, but not exclusively, in part-processedsilicon, fully processed device structures and checking forcross-contamination.

Developments in crystal growth have enabled the production of siliconwafers free from dislocation. Wafers are then chemically etched toremove damage from sawing into wafers, and subjected to high temperatureprocessing to form an oxide layer, during which defects may form withinthe device active region in the wafer and in the gate oxide, whichgenerally degrade device performance, and can lead to yield losses andreliability problems. WO 98/11425 discloses and claims an apparatus andmethod to locate and measure the electrical activity of these defects,by means of photoluminescence (PL) mapping.

Silicon wafers also suffer from the presence of metal impurities, inparticular transition metals, present as minor impurities in thematerials used, and predominantly in the chemicals used for example inthe SC1 cleaning (NH₄OH/H₂O₂/H₂O) to remove particles. A devicestructure is produced in three main steps of wafer fabrication. Duringthe first step the active device areas are created. Secondly, the fullyprocessed chip is protected with a passivation layer. Finally, one ormore layers of conducting metal are deposited at specific locations onthe wafer to facilitate electrical connection of individual circuitcomponents. These metal layers form surface wiring and are used toproduce, contact pads, metal lines or interconnects. The metal istypically chemically deposited on the wafer and unwanted regions areetched away.

Finally a thermal treatment is used to alloy the metal to the Siliconwafers to ensure good electrical connection. Traditionally Aluminum hasbeen used as the conductor material, because it has low resistivity andcan carry moderate current density.

As semiconductors get smaller and smaller, Copper is seen as a betterchoice for interconnects because it is a better conductor thanAluminium. Another advantage of Copper is that because very thin linescarry signals, the material can be placed close together making forshorter distances and faster chips with more transistors. Copper'stendency to diffuse into the surrounding chip material, poisoning thesilicon, has largely been overcome by modifying the deposition step tofirst deposit a barrier material lining the etched tracks, onto whichthe Copper is deposited, sealed by further barrier material deposition.The chip is then polished.

Copper contamination can occur from the tools and equipment involved inthe metalization process (metal deposition, etch, polishing and waferhandling). Also it is possible that inadvertent cross-contamination canoccur on the wafer backside during the Copper process. Coppercontamination on the wafer backside can be transferred to the waferrobotic handling equipment or onto metrology tools that are used forboth Copper and non-Copper processes. Copper contamination can alsooccur prior to metalization during wafer polishing, epitaxial growth orsurface cleaning.

Transition metals, which are fast diffusers in silicon, readily formdeep levels, i.e. away from the valence or conduction band edge, andalso lead to formation of both point and extended defects whicheventually lead to device failure. These transition metal impuritiesalso form recombination centres and traps which can dramatically reducecarrier lifetime and can also act as dark current generation sites, i.e.in the absence of light, charge leakage will occur. Getteringtechniques, where mechanical damage, such as abrasion, is typicallyundertaken in order to provide a damaged site, which effectively acts asa sponge soaking up impurities in the wafer, have been developed toremove transition metal impurities from the device active areas. Copperdiffuses rapidly in Silicon having faster diffusion than Gold or Iron,and can diffuse through Silicon wafers at room temperature. Cooperpoisoning of devices leads to high device leakage, dielectric breakdownand also to electrical shorts and leaky paths between conductors.

With the additional sources and metal contamination introduced duringpolishing and by residual migration, gettering techniques areinsufficient to provide the high purity required and there is a need todetect and eliminate sources of contamination.

Several techniques already exist for the detection of low level surfacecontamination in polished semiconductors and copper interconnects. Theseinclude classical metal detection techniques, which detect the physicalpresence of metals, these techniques are destructive. The mainnon-destructive chemical analysis method is Total X-ray ReflectionFluorescence (TXRF), which is a laboratory technique which only samplesa small area of the whole wafer (≈1 cm²) at a time and is very timeconsuming. Moreover the physical detection identifying metals directlyis unduly detailed and inefficient.

We have now surprisingly found that certain metals providecharacteristic photoluminescent “finger prints”, dependent on theirconcentration, when subject to photo luminescence (PL) spectroscopy, andthis can be used in a new method to scan whole wafers, identifyvariations and locate contamination for more detailed inspection.

WO 98/11425 discloses that Photoluminescence (PL) spectroscopy is a verysensitive technique for investigating both intrinsic and extrinsicelectronic transitions at impurities and defects in semiconductors. Whensilicon is excited at low temperatures with laser irradiation above theband-gap of the material, electron hole pairs are produced. Thesecarriers can recombine in various different ways, some of which giverise to luminescence. The electron hole pairs formed at low temperaturecan be trapped at impurities in silicon and they emit photonscharacteristic of this interaction, thereby giving impurity specificinformation in the photoluminescence spectra. There are a significantnumber of applications of PL spectroscopy to silicon includingcharacterisation of silicon after different processing steps,characteristic of device fabrication for example implantation,oxidation, plasma etching, the detection of point defect complexes andthe presence of dislocations. The present invention provides a newapplication of PL spectroscopy in surface metal detection.

In its broadest aspect there is provided according to the presentinvention a method for detecting surface or near surface metalcontamination in a semiconductor or silicon structure, comprisingexposing the structure or a part thereof with an excitation beam ofpredetermined wavelength and collecting luminescence from the structurein the form of a PL map characterised by a substantially uniform PLintensity level provided by the semiconductor; and inspecting the mapfor one or more regions of enhanced PL intensity, identifyingcharacteristic (near) surface metal contamination.

Preferably regions of greater PL intensity are observed as white spots,haze or mist, contrasted against the lower intensity darkersemiconductor background.

The method of the invention identifies characteristic photoluminescencefrom certain metals which have been found to provide a peak PL intensityat a given metal concentration, greater than the PL intensity at lowerand higher metal concentrations, and significantly greater than the PLintensity of the semiconductor itself. Preferably the method identifiesthe effect on photoluminescence of Copper contamination in Silicon inthe range 1×10⁹-5×10¹⁴ atoms/cm², more particularly in the range1×10⁹-5×10¹² atoms/cm², yet more particularly in the range 6×10¹⁰-4×10¹²atoms/cm².

Without being limited to this theory it is thought that certain metals,present in a certain low level concentration form a less activerecombination centre for which rate of recombination is determined bycarrier availability, and therefore provides high intensity PL ifsubject to higher injection levels according to the present invention.

The surprising nature of the invention relies on the finding that coppernot only provides a unique PL finger print which is uniquelyidentifiable at a particular concentration level, but that the coppercontamination within a wafer falls within this concentration due to itstendency for diffusion throughout silicon, whereby the uniquelydetectable level coincides with that of the low level contaminationwhich takes place as a result of contact or condensation during cleaningor polishing, or which takes place by diffusion from high levelcontamination from copper interconnects, and the like. Preferablytherefore the method is a method for detecting surface or near surfacemetal contamination or contamination trail in a wafer comprisingsemiconductor or silicon and metal functional components, selected frominterconnects and the like.

More preferably in the detection of Copper contamination from polishedCopper interconnects we have found that the method enables detection offront side contamination by diffusion from the rearside point of entryof contaminant as well as direct surface contamination. This isparticularly advantageous since it allows surface detection ofcontamination within a structure or at an opposite surface.

Preferably the method of the invention employs a sensitive rapid roomtemperature PL mapping system (called SiPHER Silicon Photo EnhancedRecombination) which produces maps or visual images of a structure, asdistinct from a spectrum or other indirect representation. This systemcan record wafer maps (up to 300 mm diameter) and then examine regionsof interest in greater detail using high resolution micro scans(approximately 1 micron resolution). Preferably excitation is by laserbeam focused to create a small excitation volume (1-2 micron diameter)and utilises a differential opto-acoustic modulator to drive the lasersource whereby the photogenerated carriers are strongly confined and thecarrier diffusion length is greatly reduced. Preferably visible laserexcitation is used in the region of substantially 532 nm whereby thephotogenerated carriers are effectively confined to the near surfaceregion for example of up to 1 micron. Preferably the luminescence isonly sampled from a small region within the overall diffusion envelopewhich further enhances the spatial resolution.

The excitation laser beam is focused to give desired intensity,accordingly laser beam power may be selected as appropriate.

Our technique can be described having regard to the followinginformation.

The intensity of the PL intensity I_(PL), as a function of the laserbeam position (x,y), is given by:I _(PL)(x,y)=k∫AR _(f)ηΔnd₃  (1)

Where k is the proportionality factor to allow for experimentalparameters such as light collection efficiency, detector quantumefficiency. A and R_(r) are correction factors for absorption losses,inside the material (A) and reflection losses at the surface (R_(r)). ηis the internal quantum efficiency of the semiconductor, Δn is theexcess carrier density and d³ is the volume of material emitting light.

The contaminants alter the recombination properties of the carrierswhich is observed as a change in intensity in the PL image. We definethe PL contrast C, as follows: $\begin{matrix}{{C\left( {x,y} \right)} = \frac{{I_{PL}(\infty)} - {I_{PL}\left( {x,y} \right)}}{I_{PL}(\infty)}} & (2)\end{matrix}$where I_(PL)(∞) is the intensity far away from the contaminant, andI_(PL)(x,y) at the position x,y.

When excess electron-hole pairs are produced in Si by above band gapexcitation, recombination can either be radiative (emit light) or nonradiative. The total recombination rate is expressed by the sum of tworates:R=R _(rr) +R _(nr)  (3)and the internal quantum efficiency, η of the semiconductor is given by:$\begin{matrix}{\eta = \frac{Rrr}{{Rrr} + {Rnr}}} & (4)\end{matrix}$

When the photoluminescence images are obtained any variations observedin the PL signal could be due to the spatial variations of both R_(rr)and R_(nr).

The recombination behaviour of the contaminant depends on the positionof the levels in the band gap (deep or shallow) and on the carriercapture cross sections. At low injection levels the recombination rateis limited by the availability of the minority carriers, whereas at highinjection levels, where the injected charge exceeds the equilibriumcarrier concentration, the recombination rate is limited by the numberof traps. Therefore increasing the injection level leads to enhancedrecombination at the contaminant.

Without being limited to this theory, it is thought that certain metalcontaminants, in particular certain diffusable metals, more particularlyCopper modifies the surface recombination lifetime τ_(s) and that thismodifies the PL intensity according to the following relation:$I_{PL} = {\left( {1 - R} \right)\frac{1}{\tau_{r}}{\int_{{Layer}{(i)}}{\Delta\quad{p(z)}\quad{\mathbb{d}z}\quad{\alpha\tau}_{eff}}}}$Where$\frac{1}{\tau_{eff}} = {\frac{1}{\tau_{s}} + \frac{1}{\tau_{n\quad s}}}$

The invention provides a photoluminescence technique which can beundertaken at room temperature and which provides information concerningmetal contamination in a semiconductor or silicon structure at a rateappropriate to industrial use and/or which enables us to visualise metalcontamination in the upper regions of the semiconductor or siliconstructure and in particular near to the surface of same.

The method used in the invention enhances non-radiative recombination ofelectron hole pairs at contaminants in a semiconductor or siliconstructure with a view to enhancing contrast in a PL image of saidsemiconductor or silicon structure so as to enhance the viewing ofcontaminants in same.

We therefore use a high injection level laser in the method of theinvention and contaminants are detected due to the local change incarrier lifetime at the surface. These characteristic contaminants aretypically observed as lightened regions at the physical position ofdefined contaminant, from which a high level contaminant source may betracked, or at the physical position of low level contaminant source.

In the method of the invention the probing volume of the laser is small(spatial resolution 0.1-20 μm, ideally 2-5 μm) and therefore localisedcontaminants have much greater effect on the measured PL intensity.Moreover the excitation is focused whereby the injected carrier densityis high (spot size of 5 cm-0.5 micron and peak or average power of 10⁴to 10⁹ watts/cm²). This greatly increases the probability ofnon-radiated recombination at the contaminant and hence physicallocation of the contaminant.

We have discovered that carrier diffusion lengths are greatly reducedunder high injection laser conditions, the result of this is that theeffective sampling depth is largely determined by the excitation laserpenetration depth which is in turn determined by the wavelength of theexcitation source. By using a short wavelength near surface contaminantscan be examined. Conversely longer wavelengths can be used to look atcontaminants deeper in the structure.

In a preferred method of the invention we use a pulsed laser excitationsource and ideally measure the luminescence images as a function oftime. This means that both depth and spatial resolution are improved andcan be used to obtain information on the carrier capture cross sections,indicating source of contaminant e.g. trails of diffusable contaminants.Time resolved measurements can also be used to measure the effectivecarrier lifetime and obtain lifetime maps.

In a further embodiment of the invention confocal optics are used toobtain depth discrimination of the contaminants by exciting a largevolume of said semiconductor with a laser and collecting images from aseries of focal planes.

In a further aspect of the invention there is provided a method for thepreparation of a metal interconnect and/or a method for the detection ofmetal contamination directly or by cross-contamination following thepreparation of a metal interconnect, comprising etching at least onetrack or via in a silicon wafer and laying down a barrier lining anddepositing a diffusable metal and sealing with a further barrier layerand polishing to produce a finished product, and exposing to anexcitation beam of predetermined wavelength, and collecting luminescencefrom the structure in the form of a PL map and inspecting for one ormore regions of enhanced PL intensity identifying potential front sideor back side diffusable metal contamination; and verifying by additionaltests.

Preferably, the method may be comprised as an in-process quality controlor as a quality control of processed structures.

In a further aspect of the invention there is provided a semiconductoror silicon structure tested for surface metal contamination according tothe method of the invention and verified for use, being contaminantfree.

According to a yet further aspect of the invention there is provided theuse of an apparatus in photoluminescence imaging of a semiconductor orsilicon structure for the detection of surface region metalcontamination.

An embodiment of the invention will now be described, by way of exampleonly, with reference to the following Figures wherein:

FIGS. 1 to 4 are PL images of localised copper contaminated siliconwafers, in which contaminant is imaged by PL contrast using the methodof the invention;

FIG. 5 is a diagrammatic illustration of an apparatus in accordance withthe invention.

Referring to the figures and firstly to FIG. 5 there is shown adiagrammatic illustration of an apparatus in accordance with theinvention.

The apparatus essentially comprises a PL imaging microscope which:towards the right hand side, comprises a bank of lasers 3-8; towards thebottom comprises a sample stage such as an X-Y table or R-Θ table;towards the left hand side comprises a microprocessor 40 and a displayscreen 39 and in the centre of the Figure there are shown variousoptical components for directing light through the system.

In the embodiment shown in FIG. 7, six lasers are provided with a viewto probing different depths in the sample. However, it is within thescope of the invention to use only one laser, or indeed to use a greaternumber of lasers. In any event, at least one of the lasers is a highintensity laser and ideally has a spot size of between 0.1 mm and 0.5micron and a power density of between 10⁴ to 10₉ watts/cm². A laserselector 16 coupled with said bank of lasers is provided so as to selectone or more lasers for use and further also to select the frequency andwavelength of the lasers.

Conventional optics, such as optical fibres 9 are used to direct lighttowards the collimator to 10 and laser beam expander 11. An apodizationplate 12 is positioned between laser beam expander 11 and beam splitter31. Beam splitter 31 directs a fraction of light from the aforementionedlasers towards sample 2 via objective 34.

An automatic focus controller 30 is provided and coupled to a piezodriven focusing stage 33. The microscope is equipped with a conventionalrotating turret 36 which is provided with at least one high numericalaperture objective for micro examination and one low numerical apertureobjective for macro examination 34,35 respectively. In addition, alsocoupled to turret 36 there is provided an optical displacement measuringsystem 38.

Cabling is provided so as to connect the automatic focusing controller30 to microprocessor 40 and also a microscope objective indexingarrangement 32 to microprocessor 40.

Downstream of beam splitter 31 there is provided as filter wheel 13 forlaser notch filters, down stream thereof there is provided a swing-asidefolding mirror 14 whose function will be described hereinafter. Alignedwith said mirror 14 there is provided a filter wheel 27 for wavelengthselection, and rearward thereof there is provided a zoom lenses attachedto a suitable CCD 2-D array detector 29.

Infinity system compensating lens 37 is provided in the optical pathforemost of cold mirror 17 which reflects light towards a further filterwheel 23 for wavelength selection and a focusing lenses 24 which isforemost of a detector 25 for UV and visible light. Detector 25 iscoupled to lock-in amplifier 26. This is used to obtain a reflectedimage of the surfaces.

Rearmost of cold mirror 17 is provided a further filter wheel 18 againfor wavelength selection, and rearmost thereof a focusing lens 22 and afurther aperture wheel 19 for pinhole selection which is providedforemost of a detector 21 for detecting the luminescence.

Both the UV and visible region detector 25 and infrared detector 21 arecoupled to lock-in amplifier 26.

Operation of the system is explained having regard to the following.

A range of wavelengths to probe different planes in the sample isprovided by several lasers (3-8). The lasers can be modulated by afrequency generator (16) so that the signal emitted from the sample (2)can be isolated from background radiation by means of the detectorsbeing synchronised to the laser modulation frequency by the lock-inamplifier (26. In a further embodiment, the range of wavelengths couldbe produced by using a tuneable laser and/or an Optical ParametricOscillator. Each laser is connected to, and aligned with, a Multi-branchoptical fibre (9) so that any or all of the lasers can illuminate thesample (2). The common end of the Multi-branch optical fibre terminatesin an optical system (10) which collimates the emerging light. Thisoptical system is aligned with a beam expander 911) which matches thelaser beam's diameter to that required by the microscope objectives(34,35) above the sample (2). The expanded beam then passes through anapodization plate (12) which distributes the optical energy evenly overthe beam area.

The expanded and apodized beam is reflected by a beamsplitter (31) andpasses to the microscope objectives (34 and 35). The beam is focused bya microscope objective (34 or 35) on to the sample. In the micro modethis objective is selected to focus the beam to a diffraction limitedspot size. A rotating turret (36), operated by an indexing mechanism(32), permits the objective to be changed for the macro mode where alarger area of the sample can be illuminated. In a further embodimentthe apodization plate (12) can be removed so that the spot for the micromode can be made smaller to allow higher injection levels.

An optical displacement sensor (38) measures the distance to the sampleand, by means of a feedback loop through the antifocus controller (30),maintains the correct spacing by means of the piezo actuated focusingstage (33).

The Photoluminescence signal from the sample is collected by themicroscope objective (34) (in the micro mode) and transported backthrough the beamsplitter (31) and a notch filter in the filter wheel(13) which contains notch filters matched to the range of laserwavelengths. The notch filter removes any reflected laser light, passingonly the Photoluminescence signal.

The folding mirror (14) is swung out of the beam allowing the signal topass to the tube lens (37), which may be incorporated to compensate forany infinity microscope objectives which may be used, and on to the coldmirror (17). This component reflects those wavelengths below a selectedcut off point (approximately 700 nm) to the focusing lens (24) whichfocuses the signal into the detector (25). A filter wheel (23) in frontof the detector focusing lens (24) contains filters to isolate selectedwavelength bands.

The portion of the Photoluminescence signal lying in the wavelengthrange above the cut-off point passes through the cold mirror (17) and issimilarly focused by the lens (22) into the detector (21). This signalalso passes through a filter wheel (18) containing filters to isolateselected wavelength bands.

A series of pinholes of different diameters are contained in an aperturewheel (19) positioned in front of the detector (21). This aperture wheelcan be moved axially by the piezo actuator (20) so that the pinholes canbe positioned confocally with the desired image plane. By this means,planes at different depths in the sample (2) can be imaged to provideaccurate depths information.

The electrical signal from the detectors (21,25) is fed to the lock-inamplifier (26) where it is synchronised with the modulation frequency ofthe laser (3-8) by means of a reference signal from the frequencygenerator (15). The electric signal is then fed to the central processor(40) for analysis. The PL image is obtained by raster scanning thestage. Alternatively optical scanning using galvo mirrors may beemployed.

In an alternative micro mode of operation, the folding mirror (14) isswung into the beam of the Photoluminescence signal. The diverted signalpasses through a filter wheel (27), which contains filters to isolateselected wavelength bands, and into the zoom lens (28). The zoom lensallows different magnifications to be used in imaging the illuminatedspot on the sample (2) on to the CCD two dimensional array (29). Thisallows the illuminated area of the sample (2) to be imaged at differentresolutions. The electrical signal from the CCD array is fed to thecentral processor (40) for analysis.

Using the aforedescribed apparatus investigations were undertaken inorder to visualise metal contamination in semiconductors and the resultsof these investigations are shown in FIGS. 1-6. The images are uniqueand cannot be obtained by any other method at room temperature.Generally, it can be seen that use of the equipment enables localisationand characterisation of certain PL characteristic metal contaminants insemiconductors. This enables one to more efficiently screen wafers, andin particular microprocessors and copper interconnects, for devicefabrication and so safeguard against the production of defectivesemiconductors.

It can therefore be seen that the invention provides the use of anapparatus and a method for imaging certain metal contamination in asemiconductor or silicon structure which enables the contaminants to beimaged so that the density and spatial distribution of same can bedetermined.

EXAMPLES

Method for Imaging

Silicon wafers were contaminated with copper in the range 1×10⁹-5×10¹²atoms/cm² or contaminated wafers were obtained. TXRF was used to confirmor determine the contamination levels. Control wafers were obtainedhaving no contamination.

The wafers were excited using 532 nm laser excitation, both whole wafermaps and micro scans were obtained, from which the average PL signal wascalculated. PL maps were obtained, FIGS. 1 to 4, which comprise imagesof the wafers, with lighter regions indicating the position of Coppercontamination from copper in the above concentration range. Controlwafers showed no lighter regions.

Example 1

Contaminated and control silicon wafers were imaged, as described above,and the results are shown in FIGS. 1 to 4. Inspection of the imagesrevealing lighter contrast regions at the location of intentionalcontamination, and not in the control wafers, confirmed that the Coppercontamination produced higher intensity PL which could be detected byobservation, to indicate the nature and location of the contaminantintroduced.

Example 2

Polished silicon wafers were imaged (not shown), by the method asdescribed above. Inspection of the images indicated a white haze whichsuggests copper contamination. As a result of a positive indication ofcontamination, in depth examination and TXRF could then be performed toconfirm the nature of contaminant, being copper in the aboveconcentration range, whereby the nature and location of contaminationmay be ascertained, in this case being found to be the polishing toolitself. The polishing tool can then be treated or replaced to eliminatethe source of contamination.

Example 3

Cleaned silicon wafers from the SC1 cleaning step were imaged (notshown), as described above. Inspection of the images indicated a whitehaze, which suggests copper contamination. As a result of a positiveindication of contamination, in depth examination and TXRF was thenperformed to confirm the nature of contaminant, being copper in theabove concentration range, whereby the nature and location ofcontamination may be ascertained, in this case, being the cleaningmaterials used. The materials can then be treated or replaced toeliminate the source of contamination.

Example 4

A PL map is recorded from the wafer using 532 nm laser excitation,showing the entire wafer (near) surface, using the method as describedabove. The map is inspected for presence of regions of increased PLintensity observed as white spots or cloudy regions compared to thegrey/black “background” Si PL.

Spots or regions are detected suggesting possible Copper contaminationat the image site of increased PL intensity, which are then inspected athigh resolution to reveal the nature and location (depth) ofcontamination indicating front or back surface contamination or waferedge contamination which in turn infers the source of contaminationbeing cleaning, polishing or interconnect formation Processing ormetrology tools/or cleaning fluids may then be treated or replaced toeliminate the source of contamination or barrier layer depositionquality improved to eliminate the source of copper diffusion.

1. A method for detecting surface or near surface metal contamination orcontamination trail in a wafer comprising semiconductor or silicon andmetal functional components comprising interconnects and the like, saidmethod comprising exposing the structure or a part thereof to visiblelaser excitation of predetermined wavelength in the region ofsubstantially 532 nm whereby the photogenerated carriers are effectivelyconfined to the nearest surface region of, for example, up to 1 μm andcollecting luminescence from the structure in the form of a PL mapcharacterised by a substantially uniform PL intensity level provided bythe semiconductor, and inspecting the map for one or more regions ofenhanced PL intensity, identifying characteristic surface or nearsurface metal contamination.
 2. The method of claim 1 wherein regions ofgreater PL intensity are observed as white spots, haze or mist,contrasted against the lower intensity darker semiconductor background.3. The method of claim 1 which is adapted to identify a peak PLintensity characteristic of the affect on photoluminescence of coppercontamination in silicon in the range of 1×10⁹ to 5×10¹⁴ atoms/cm². 4.The method of claim 3 wherein copper contamination is in the range 1×10⁹to 5×10¹² atoms/cm².
 5. The method of claim 3 wherein coppercontamination is in the range 6×10¹⁰ to 4×10¹² atoms/cm².
 6. The methodof claim 1 applied to the detection of copper contamination frompolished copper interconnects including both front side contamination bydiffusion from the rear side point of entry and direct surfacecontamination.
 7. The method of claim 1 wherein a sensitive rapid roomtemperature photoluminescence mapping system is employed which producesmaps or visual images of a structure as distinct from a spectrum orother indirect representation.
 8. The method of claim 1 whereinexcitation is by a high intensity laser beam.
 9. The method of claim 8wherein the beam is focused to create a volume (1-2 μm diameter) andutilises a differential opto-acoustic modulator to drive the lasersource whereby the photogenerated carriers are strongly confined and thecarrier diffusion length is greatly reduced.
 10. The method of claim 8wherein the probing volume of the laser is restricted to a spatialresolution of 0.1 to 20 μm.
 11. The method of claim 10 herein theprobing volume is restricted to a spatial resolution of 2 to 5 μm. 12.The method of claim 8 wherein the excitation is focused such that theinjected carrier density is high to produce a spot size of 5 cm to 0.5μm and a peak or average power of 10⁴ to 10⁹ watts/cm².
 13. The methodof claim 1 comprising the use of a pulsed laser excitation source andthe measurement of luminescence images as a function of time.
 14. Themethod of claim 1 in which confocal optics are used to obtain depthdiscrimination of the contaminants by exciting a large volume of saidsemiconductor with a laser and collecting images from a series of focalplanes.
 15. A method for the preparation of a metal interconnect and/ora method for the detection of metal contamination directly or bycross-contamination following the preparation of a metal interconnect,comprising etching at least one track or via in a silicon wafer andlaying down a barrier lining and depositing a diffusible metal andsealing with a further barrier layer and polishing to produce a finishedproduct, and exposing to an excitation beam of predetermined wavelength,and collecting luminescence from the structure in the form of a PL mapand inspecting for one or more regions of enhanced PL intensityidentifying potential front side or back side diffusible metalcontamination; and verifying by additional tests.
 16. The method ofclaim 15 comprised as an in-process quality control or as a qualitycontrol of processed structures.